Synergistic effects of silver ions and metformin against enterococcus faecalis under high-glucose conditions in vitro

BACKGROUND: This study aimed to evaluate the synergistic antibacterial activities of silver ions (Ag(+)) and metformin hydrochloride (Met) against Enterococcus faecalis (E. faecalis) under normal or high-glucose conditions. RESULTS: The minimum inhibitory concentration, minimum bactericidal concentration, growth curves, and colony-forming units were used to evaluate the antibacterial effects of Ag(+) and Met on planktonic E. faecalis in Brain Heart Infusion broth with or without additional glucose. The influences of Ag(+) and Met on four weeks E. faecalis biofilm on human dentin slices was also tested. Cytotoxicity was tested on MC3T3-E1 osteoblastic cells using CCK-8 assays. The results indicated that E. faecalis showed higher resistance to drug treatment under high-glucose conditions. Ag(+) (40 μg/mL) plus Met (3.2% or 6.4%) showed enhanced antibacterial activities against both planktonic E. faecalis and biofilm on dentin slices, with low cytotoxicity. CONCLUSIONS: Met enhanced the bactericidal effects of Ag(+) against both planktonic and biofilm E. faecalis under normal or high-glucose conditions with low cytotoxicity. Further molecular studies are needed to be conducted to understand the mechanisms underlying the synergistic activity between Met and Ag(+). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12866-021-02291-2.